Cobia (Rachycentron canadum) is a migratory pelagic species that is
found in tropical and subtropical seas of the world, except in the
central and eastern Pacific Ocean. The United States ranks third in
total commercial production of cobia, however, recreational landings
generally exceed commercial landings by an order of magnitude (Shaffer
and Nakamura, 1989). In the western Atlantic Ocean, cobia migrate to
Chesapeake Bay in spring and summer to spawn, and the productive waters
of the Bay are believed to constitute important foraging grounds (Joseph
et al., 1964; Richards, 1967). Cobia are known to move to areas of high
food abundance, particularly crustacean abundance (Darracott, 1977).

Recent feeding studies in the northern Gulf of Mexico and off North
Carolina have reported geographic differences in cobia diet and have
indicated that the relative importance of fishes versus crustaceans is
variable and that cephalopods constitute the least significant prey
items. In the northern Gulf of Mexico, Franks et al. (1996) reported
that fish (primarily anchovies, Anchoa sp.) dominated (% index of
relative importance [IRI], Pinkas et al., 1971) the diet of juvenile
cobia (236-440 mm FL). Meyer and Franks (1996) reported crustaceans
(primarily portunid crabs) occurred in 79.1% of stomachs and represented
77.6% of total prey items consumed by cobia (373-1,530 mm FL) in the
northern Gulf of Mexico. Fish (primarily hardhead catfish, Arius felis,
and American eel, Anguilla rostrata) increased in importance with
increasing cobia size. Fish were found in 58.5% of all cobia stomachs
(20.3% of total prey) but occurred in 84.4% of stomachs of cobia
1150-1530 mm FL (Meyer and Franks, 1996). In contrast, Smith (1995)
observed decreased importance of teleosts in the diet of cobia (39-142
cm FL) in North Carolina. Elasmobranch fishes and portunid crabs
dominated the diet of cobia >9 kg (Smith, 1995).

Tag-recapture data collected between 1995 and 1999 document
localized movement of cobia within lower Chesapeake Bay during summer,
as well as the return of individual cobia to specific locations or
general regions of the lower Bay in subsequent summers. (1) Although
Chesapeake Bay is an important destination for migrating cobia, feeding
habits of cobia in the Bay have never been thoroughly examined. Our
study documents cobia feeding habits in Chesapeake Bay and compares
findings with similar cobia studies from North Carolina and the northern
Gulf of Mexico.

Methods

Cobia were sampled opportunistically at marinas and fishing
tournaments in lower Chesapeake Bay between June and July 1997. Intact
stomachs were removed by cutting above the cardiac sphincter (esophagus)
and below the pyloric sphincter (large intestine). Stomachs were
labeled, bagged, transported on ice to the VA Institute of Marine
Science, and examined in relatively fresh condition. An incision was
made along the longitudinal axis and the contents of stomachs were
emptied onto a 500-[micro]m mesh sieve for rinsing and sorting. Contents
were blotted dry on paper towels before counts, displaced volumes (1-L
graduated cylinder), and identifications to the lowest possible taxon were made. When possible, carapace widths (mm) of crabs were measured
with calipers. An index of relative importance (IRI) for all prey items
combined was calculated with the formula (% Number + % Volume) x (%
Frequency), as described by Pinkas et al. (1971) and subsequently used
by Smith (1995), Meyer and Franks (1996), and Franks et al. (1996).
Frequency (%F) was the percent of all stomachs that contained food.

Results and discussion

Stomach contents from 114 adult cobia (37-141 cm FL) were examined.
Seventy-eight stomachs (68%) contained at least one identifable, nonbait
prey item. One species of bivalve, one species of hydroid, six species
of crustacean, one elasmobranch, and 16 species of teleost were observed
(Table 1). Mean volume of prey per stomach was 150.6 mL (range: 2-680
mL). Mean number of different prey per stomach was 1.9 species (range,
1-5 species). Blue crab (Callinectes sapidus) and lady crab (Ovalipes
ocellatus) dominated the diet of cobia.

Index of relative importance (IRI) values for blue crab (5257) and
lady crab (3665) were two orders of magnitude higher than IRI values for
other prey items. Larger cobia consumed greater volumes of crab (Fig.
1). Additionally, larger cobia consumed larger (sublegal, <13 cm)
blue crabs, although similar-size lady crabs were consumed by all sizes
of cobia (Table 2). Atlantic croaker (Micropogonias undulatus),
hogchoker (Trinectus maculatus), and fish and crab remains constituted
the top food items volumetrically after portunid crabs. Although
relatively high volumes were observed, IRI values for Atlantic croaker
and hog-choker were low owing to infrequent occurrence of these items.
High volume of crab remains was consistent with high volumes, counts,
and frequency of occurrence of portunid crabs in cobia stomachs. Fish
remains likely resulted from finfish bait (predominantly menhaden,
Brevoortia tyrannus, and Atlantic croaker, Micropogonias undulatus).
Crab and fish remains not identifiable to family and finfish bait were
excluded from IRI calculations.

[GRAPHIC OMITTED]

Consumption of blue crab was greatest along the western shore of
the Bay and least at the mouth of the Bay. Conversely, consumption of
lady crab was greatest at the mouth of the Bay and least along the
eastern shore of the Bay (Fig. 2A). A greater percentage of lady crabs
were consumed by male cobia than by female cobia, whereas a greater
percentage of blue crabs were consumed by female cobia (Fig. 2B). IRIs
for blue crab and lady crab were similar in June, but dramatically
different in July. In July, IRI for blue crab was twice as high as that
for lady crab. Prey availability and habitat utilization were likely
responsible for determining location, sex, and within-season differences
in composition of portunid crabs consumed by cobia. June samples were
predominantly male cobia collected from the Bay mouth and eastern shore
of the Bay, whereas July samples were predominantly female cobia
collected from the western Bay shore.

[GRAPHICS OMITTED]

Cobia feeding habits in lower Chesapeake Bay were more similar to
feeding habits reported for cobia from North Carolina (Smith, 1995) than
to feeding habits of cobia from the northern Gulf of Mexico (Franks et
al., 1996; Meyer and Franks, 1996). Portunid crabs dominated the diet of
cobia in Chesapeake Bay and North Carolina (Smith, 1995). Elasmobranch
fishes were consumed exclusively by large cobia in Chesapeake Bay and
North Carolina (Smith, 1995). In North Carolina (Smith, 1995), cobia fed
on stingrays (Dasyatis sp.) and smooth dogfish (Mustelus canis). Cownose
ray (Rhinoptera bonasus), previously unreported as a food item of cobia,
was the only elasmo-branch consumed by cobia in Chesapeake Bay. Cownose
ray was observed from eight cobia stomachs; however, only the jaw plates
(22-49 mm width) remained; thus, the IRI for cownose ray is likely
underestimated. Flatfishes and syngnathids represented the teleosts most
frequently consumed by cobia in Chesapeake Bay and North Carolina.
Hogchoker (Trinectes maculatus) was more frequently consumed by cobia in
Chesapeake Bay (our study) than in North Carolina (Smith, 1995).
Blackcheek tonguefish (Symphurus plaigusa) was regularly consumed by
cobia in North Carolina (Smith, 1995) but was absent from cobia stomachs
in Chesapeake Bay. In Chesapeake Bay, Syngnathus sp. (pipefish) and
Hippocampus sp. (seahorse) were only important in the diet of smaller
cobia, whereas in North Carolina, these fishes were important in the
diet of all cobia (Smith, 1995).

Cobia in our study predominantly consumed benthic and epibenthic
prey items, most notably portunid crabs. Feeding studies on other large
predatory fishes in Chesapeake Bay during the summer months, such as
bluefish, Pomatomous saltatrix, and weakfish, Cynoscion regalis, reveal
that these species consume prey items associated predominantly with
pelagic food webs (Hartman and Brandt, 1995a, 1995b). Striped bass,
Morone saxatalis, a large predator present in Chesapeake Bay year-round,
is also reported to feed predominantly on pelagic fishes (Hartman and
Brandt, 1995a, 1995b; Walters, 1999). During the summer, portunid crabs
were consumed by bluefish, weakfish, and striped bass in Chesapeake Bay;
however, portunid crab consumption was typically less than 5% of total
prey items present in the stomachs of these fishes (Hartman and Brandt,
1995a, 1995b; Walters, 1999). Red drum, Sciaenops ocellatus, a large
predator that uses Chesapeake Bay between spring and fall is also
reported to selectively consume portunid crabs in estuarine environments
in the Gulf of Mexico (Scharf and Schlict, 2000); however, feeding
habits of red drum in Chesapeake Bay have not been documented. Although
other large predators in Chesapeake Bay occasionally consume portunid
crabs, cobia is the only large predator in Chesapeake Bay for which
selective consumption of portunid crabs is documented.

Stomachs were collected in conjunction with a study on cobia
reproduction in Chesapeake Bay. We thank Susan Crute, Jason Romine, and
Holly Simpkins for technical assistance. Donnie Wallace and Harry
Johnson Jr. provided access to their facilities at Wallace's Bait
and Tackle, Fox Hill, VA. We thank the recreational and commercial
fishermen who allowed their catches to be sampled. This project was
supported by grants (RF96-8, RF97-9) from the Virginia Marine Resources
Commission upon recommendation by the Recreational Fishing Advisory
Board.